For Cards View, this can be done with css:

.card-container {
    background-color: var(--tag-color) !important;
}

To make just the card text have the tag colour:

.card-container {
    color: var(--tag-color) !important;
}

Help/Troubleshooting Info, Profile Folder, Open Folder, close TB, create a new folder named chrome, create a new document in chrome with a text editor, name it userChrome.css, Save as type: All files *.*, copy in the above code. Double-click toolkit.legacyUserProfileCustomizations.stylesheets to true in Settings/General/Config. editor, restart TB.

Online css generator

video on how to create a css file (Firefox and TB)

OK thanks, but I find it a bit complicated for me...

I don't understand why this feature, active in the previous version of Thunderbird, has now been eliminated. It was convenient for a quick glance at messages 🙄

The css is necessary for Cards View, which displays a small card icon with the colour of the tag. Table View shows the list with coloured text by default.

The procedure was a bit complex but I finally solved it, thanks. 😉

For Cards View, this can be done with css:

.card-container {
    background-color: var(--tag-color) !important;
}

To make just the card text have the tag colour:

.card-container {
    color: var(--tag-color) !important;
}

Help/Troubleshooting Info, Profile Folder, Open Folder, close TB, create a new folder named chrome, create a new document in chrome with a text editor, name it userChrome.css, Save as type: All files *.*, copy in the above code. Double-click toolkit.legacyUserProfileCustomizations.stylesheets to true in Settings/General/Config. editor, restart TB.

Online css generator

video on how to create a css file (Firefox and TB)

Thank you very much for this. I'll give it a go and have a play with it. I already have a Chrome file set up for other purposes.

Sorry, I'm still not sure because I don't understand how the code needs to be modified:

what do I do with the

var(--tag-color)

line? How do I link that to a particular tag or colour? Could you give me an example please. Lets say I use the 'Important' tag which may be red.

Once I know how to do that I can create custom tags with custom colours and run a filter for each account.

Thanks again!

The colour statement means that whatever colour is defined for the tag will be applied to the text or background. For the Important tag, the default colour is red, so that will be the colour of the text or background. See the attached pictures. You can of course define specific colours for each tagged message, independent of the tag colour.